There are many stories and opinions on what oil is best and how it affects your engine. One of the most common discussions is whether synthetic oil will give you the best benefit for your money. There are many myths about motor oil and engine longevity and I will try to break them down using real world data.
The Manufacturing Process and The Environment
We’ll start off from how the oil is created and discuss the environmental impact. One of the most common myths is that synthetic oil is better for the environment because it is not derived from petroleum. This is simply not true as the chemicals used to form synthetic oil are just as harmful as the petroleum products used to create conventional oil. The only way to bring an environmental benefit from synthetic oil is to extend the oil change interval so less oil changes are performed therefore less oil gets dumped every year. This should work in theory but is not always the case, especially with old school gearheads.
The Oil Change Interval
The oil change interval or OCI is the recommended mileage interval between oil changes. There is a large debate on when you should change your oil and the classic consensus has always been that it should be done every 3 months or 3000 miles. This is the main point of contention when discussing the cost and environmental benefits of synthetic oil. In my area, the current specials show 5 quarts of conventional at $13 5 quarts of full synthetic at $22. If your car takes exactly 5 quarts and you do a 3000 mile OCI then your cost per 1000 miles on the conventional is $4.33. In order to reach the same cost with the synthetic you would need to extend your OCI to 5080 miles.
Should You Switch
If you run conventional now and are thinking of switching but are afraid of the cost or extending your OCI, there is no need to worry. The 3000 mile oil change is quickly becoming outdated and with synthetics a 5000 miles OCI is no issue as some manufacturers that use synthetic factory fill go even up to 15000 miles. The 3000 mile oil change has now become a marketing tactic for lube shops and service stations. You can safely run synthetic to 5000 miles in most cars and get all of the benefits at the same cost.
There is a small subset of cars where a 3000 mile oil change may still apply. This is a very small percentage of cars that drive under “severe” conditions. This may apply to vehicles that drive in dusty or dirty environment or in extreme temperatures that may affect the oil. There are also cars where there may be drawbacks from using synthetic that outweigh the benefits. These cases happen on older cars that may have some leaky seals that are manageable with conventional but start to leak more when switching to synthetic. This may be an issues but updated seal material has been released for many older engines and the issues can be fixed in most cases.
Switching Back and Forth
In the past, many manufacturers warned about switching back and worth or mixing the two types of oil saying it could be dangerous for engines. This is absolutely not true as even the manufacturers have started mixing the oil and creating the synthetic blend, which is a mixture of the two oils. I consider the blend the same as conventional and don’t buy the synthetic blend unless it is a screaming deal or the same price point as conventional as it is not regulated and could be 5% synthetic and 95% conventional.
In the end, the first recommendation to follow is what your manufacturer suggests, especially if the vehicle already comes filled with synthetic oil. I am not a huge fan of any interval of 10000 miles or above as I have previously seen some GM 3.6L engines that liked to stretch chains with the longer OCI so I tend to keep a 5000 or 7500 mile interval for most of my vehicles. If you have always used conventional oil and don’t have any leaks at this time then by all means switch to a synthetic oil and feel free to extend your interval to 5000 miles.